128 C. M. Jackson. 



month. The dotted lines on the right indicate, for convenience of 

 comparison, the relative growth of the corresponding parts between 

 birth and adult life, based chiefly upon observations by Meeh (31). 

 The method followed in dividing the body was to separate the head 

 from the trunk by a plane passing just below the mandible anteriorly, 

 and just below the cranium posteriorly. The neck is therefore in- 

 cluded in the trunk. The upper extremities were separated from 

 the trunk by an approximately sagittal plane through the shoulder 

 joint, and the lower extremities by an oblique plane through the 

 hip joint, parallel to Poupart's ligament. As it is impossible to 

 pass these planes always in exactly the same way, the measurements 

 on different specimens are not exactly comparable to each other, 

 though the error is comparatively small. There is also a certain 

 loss of blood, etc., especially in the case of the fresh specimens. 



Head. 



As may be seen in Table II, the head in the His models of embryos 

 in the latter half of the 1st month forms from 34 per cent to 39 per 

 cent of the entire body volume. Table IV and the "Head" curve in 

 Fig. 2 show that the head reaches its maximum relative size, about 

 45 per cent of the total body volume, during the 2d month. There- 

 after it declines gradually in relative si'ze, forming only about 26 per 

 cent or 27 per cent of the total body at birth. 



His (23) from a study of the profile areas in embryos of the 1st 

 and 2d months, concluded that the head is at first relatively small, 

 increasing from about 30 per cent in the latter part of the 1st month 

 to 56 per cent of the total body at the end of the 2d month. He 

 thought that the head and trunk during this period are in a sort of 

 race for supremacy, first one being larger, then the other. Profile 

 areas, however, lacking the third dimension, are not necessarily in 

 the same proportion as the volumes. His calculates the profile areas 

 of the head in his embryos 4 mm., 5 mm. and 7.5 mm. to be 32.3 per 

 cent, 30.7 per cent and 30.6 per cent of the total body; but I find 

 in his models of these same embryos the volume of the head to be 

 respectively 34.9 per cent, 38.7 per cent and 36.6 per cent of the 



